Human immunodeficiency virus (HIV/AIDS) has become the leading infectious cause of death worldwide surpassing malaria and tuberculosis. WHO AIDS Epidemic Update, data for December 2002, lists 3.1 million deaths and 42 million people currently living with AIDS. The need for new therapeutic agents with better efficacy is evident. Dideoxy nucleosides are an important group of antiviral compounds (16, 26, 27). A member of this group, 3′-Azido-3′-deoxythymidine (AZT, Retovir, Zidovudine) was the first drug approved for the treatment of HIV. Its dose limiting adverse effect is myelosuppression (14, 36, 39), which may be worsened by the concurrent administration of other drugs that cause bone marrow suppression or that are hepatically metabolized. 2′,3′-Didehydro-3′-deoxythymidine (D4T, Stavudine, Zerit) was then approved because of better bioavailability and lower acute toxicity (1). D4T is limited by a long-term delayed toxicity, peripheral sensory neuropathy (4) which is related to mitochondrial damage (3, 5, 6, 13, 18, 22, 30, 33, 34). 2′,3′-Dideoxyinosine (ddI, Didanosine, Videx) and 2′,3′-dideoxycytidine (ddC, Zalcitabine) are dideoxynucleoside anti HIV compounds that also have peripheral neuropathy as their leading adverse effect. In the search to find anti-HIV nucleoside analogs that had less neuropathy, many classes of compounds were synthesized and assessed for their antiviral activity and cytotoxicity including their impact on mitochondrial DNA. Dideoxynucleosides in the unnatural L conformation represented by β-L-2′,3′-dideoxy-3′-thiacytidine (3TC, Lamivudine), its 5-fluoro analog (FTC, Emtricitabine) and β-L-2′,3′-dideoxy-2′,3′-didehydro-5-fluorocytidine (LFd4C, Elvucitabine), have been shown by us (2, 11, 12, 23-25) and others (8, 9, 15, 37) to have good antiviral activity and low mitochondrial toxicity. However, even with compounds relatively non-toxic to mitochondria there is a lack of a durable response. This condition can be caused by either the rapid emergence of resistant virus or by host changes that cause differences in drug metabolism (10, 19, 35).
One approach to combat this problem is to develop compounds with less toxicity and lack of cross-resistance to other antiviral drugs. When used in combinations these compounds may decrease the dosage of existing drugs needed to achieve the same antiviral effect with less toxicity. Furthermore, these compounds could even delay the onset of resistance, which could be based on the decreased viral load during treatment. In the search for a new antiviral compounds, others have looked at 4′-substituted dThd analogs (29) (32), while we synthesized a series of 4′-substituted D4T analogs. Screening revealed the 4′-ethynyl D4T to be the most active among those tested (17). In the studies described within we describe the structure activity relationship of this class of compounds and characterize 4′-ethynyl D4T in more detail with respect to its mode of action against HIV and its interaction with key cellular enzymes that mediate its activity.